I 10 



Cincinnati Society of Natural History. 



V. 



General Conclusions and Summary. 



From the preceding field and laboratory study of the 

 granitic rocks exposed along the northern bank of the Sus- 

 quehanna River in Maryland, the general conclusion is reached 

 that, in spite of an often pronounced foliated structure, they 

 are truly igneous masses, similar to those occurring in the 

 crystalline Piedmont belt farther south, whose eruptive origin 

 is more apparent. Granites, showing a considerable range 

 in chemical and mineralogical composition, are widely dis- 

 tributed through the ancient crystalline belt along our Atlantic 

 border. That these rocks vary greatly in age, is seen from 

 the fact, that while some of them have been extremely meta- 

 morphosed by dynamic processes, others are almost entirely 

 unchanged. 



The proofs of the eruptive character of the youngest, and 

 hence the least altered, of these granite masses, as well as of 

 many more basic areas occurring with them, are altogether 

 convincing. Branching dykes and apophyses penetrating the 

 adjoining schists and gneisses, large irregular fragments of 

 the enclosing rocks torn off and wholly included within the 

 eruptives, as well as pronounced evidence of contact action 

 around them, bear ample witness to their igneous origin. In 

 the case of older rocks of similar character, such decisive 

 proofs of eruptive origin must necessarily become less and 

 less distinct in proportion as the rocks have become foliated 

 and metamorphosed. We are, therefore, compelled, when 

 dealing with the more ancient of these intrusive rocks in our 

 Pre-Cambrian belt, to reason somewhat from analog)', asso- 

 tion and mode of geological occurrence, reconstructing as 

 well as may be from evidence now at hand what was probably 

 the original condition of such masses. 



In the case here under consideration, the rocks north of 

 the staurolitic mica-schist belt have been least altered, and 



